There are thousands of reference books about drawing faces, but only the good ol' Illustration Art blog will talk to you about drawing the backs of heads.
Most people sitting in an auditorium will ignore the rows and rows of dark, featureless, identical ovals in front of them. Nothing to see, right?
But recently I had the privilege of viewing the sketchbooks of illustrator William A. Smith and was impressed by the number of times he sat in large audiences and studied the backs of the heads in front of him. Looking at his insightful sketches, I realized that a subject I thought was uniform and uninteresting was really overflowing with variety and choices. People slouched in different ways. Their shoulders were uneven. They cocked their heads in various postures. And Smith's razor sharp eyes caught it all.
I have only a few isolated examples here, but Smith did some some superb detailed drawings containing multiple figures at what would otherwise have been a pretty dull lecture. They are in the collection of the Michener Museum.
Robert Fawcett is another illustrator who found challenges in the backs of people's heads. He never drew on automatic pilot.
If you study these heads with the same open eyes that Fawcett employed to view the originals, you'll recognize that no two are the same. Fawcett came up with some surprisingly wild and jagged ways to depict a subject the rest of us never even notice: